Frequency conversion circuits



4 Sheets-Sheet l May 23, 1939. R. L. MILLER FREQUENCY CONVERSION CIRCUITS Filed July 31, 1937 May 23, 1939. mLLER 2,159,595

FREQUENCY convnns'lon c'incun's Filed July 31, 1937 4 Sheets-Sheet 2 FIG...?

INVENTOR RAM/LL E R ATTORIVEV May 23, 1939. MlLLER 2,159,595

FREQUENCY CONVERSION CIRCUITS Filed July 51, 1937 4 Sheets-Sheet 3 85 88 4000 w 'x g g W I I i-W 98 gk mm v: 10a

FILLED INVENTOR RL MIL L ER BY if ATTORNEY Patented May 23, 1939 FREQUENCY. CONVERSION CIRCUITS Ralph L. Miller, Bloomfield, N. J., alaignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 31, 1927, Serial No. 150,008

of modulating current being connected to the other conjugate diagonal 8, l of the bridge. The copper-oxide rectifier units 8, l are poled so that each is conductive in the direction toward the common terminal 6, and the other rectifier units H), II are poled so that each is conductive away from the common terminal 1. The primary winding of the input transformer 2 is connected to a source (not shown) of the base frequency h to be converted, and a filter I2 is connected -between the secondary winding of the output transformer I and the output terminal of the circuit. The source of modulating waves applied to the conjugate terminals 6, l of the modulator bridge is the regenerative circuit I3 comprising the filter network l2 and the one-way amplifier ll of gain t having its input connected across the output of filter l2 and its output connected across the conjugate terminals 8, l of the bridge I through the transformer IS. The gain a of the amplifier It in the regenerative circuit is seiected so as to provide the required stability. As indicated, a frequency multiplier l6, indicated by the box so labeled in Fig. 1A, would be included in the regenerative circuit l3 where it is desired to secure a fractional frequency having a denominator larger than 2.

Now if the input frequency 11 is applied to the input of the second order modulator in the system of Fig. l and any frequency ,f: in the output thereof is selected by the filter l2 and fed back through the amplifier i4 and transformer II to the conjugate terminals 6, I of the modulator, the two frequencies 1; and I: will combine in the modulator to produce the two side-band frequencies fiif: which are at a certain loss with respect to In. If the amplification a is greater than the side-band loss plus the loss provided by the filter II, the side-band frequencies will be fed back and will be applied to the conjugate terminals 8, 9 of the modulator I but at a greater amplitude. If the following arbitrary case is set up llClaims.

The invention relates to the production of alternating current waves by frequency conversion methods.

An object of the invention is to convert alternating current waves of one frequency into alternating current waves of a different frequency.

A more specific object is to produce alternating current waves of desired frequencies which are accurate fractions of a given base frequency.

These objects are attained in accordance with the invention by frequency conversion circuits utilizing a process which may be termed regenerative modulation. Regenerative modulation is produced in general by feeding back the output of a balanced type modulator to the balanced or conjugate input thereof through a selective network, such as a filter, and an amplifier of fixed gain t. Such a circuit is stable and of a non-oscillatory nature as long as the loss due to the balanced condition and the network or filter is greater than the gain a of the amplifier.

The frequency conversion circuits of the invention employing this process, to be described hereinafter, may be used to produce electrical waves which are exact fractional ratios of a given frequency applied to the input, and which will follow amplitude and frequency variations in the applied waves over quite wide ranges, these circuits having exceptional frequency stability and efliciency in operation.

The various features and advantages of the circuits of the invention will be brought out in the following detailed description thereof when read in connection with the accompanying draw- 5 ings of which:

Fig. 1 shows schematically a circuit which illustrates the basic principles of the invention; Fig. IA shows a modification of this basic circuit, and

Figs. 2 to 9 show schematically in greater detail frequency conversion circuits embodying different modifications of the invention.

The fundamental concept of regenerative modulation as applied to frequency conversion may be described by referring to Fig. 1.

The frequency converter circuit of Fig. l includes a balanced modulator I of the second order type, such as that disclosed in F. A. Cowan Patent No. 1,959,459, consisting of four copper-oxide rectifier units connected in a Wheatstone bridge which is the case where I: will sustain itself, it

will be found that as indicated.

In the general case for this type of circuit depending upon the order of modulation used where n and m are integers depending upon the order of modulation. For the case of third order modulation, where a third order instead of a second order modulator is used,

n=l, m=2 or n=2, m=l

and

In the case where the frequency multiplier i6 of Fig. 1A is used in the regenerative circuit IS, the following equations may be set up;

where is is the frequency in the output of the multiplier and r is the factor by which the feedback frequency f: is multipled.

Solving these two equations simultaneously gives n 9 2 1 i i 10 s 1 i l I If the input wave is represented by the equation ey =A cos wi (11) and the frequency component is at the input of the modulator is represented as e =B cos g-Hi (12) then the two side-band outputs which are obtained are given by In general, the frequency of interest is the frequency the other frequency being eliminated by the filter I2 in the regenerative circuit. Since the input to the modulator from the regenerative circuit is obtained from the side-band output in the case of a self-sustaining wave, then the condition for the phase relation of the regenerative component at the input and output of the moduwhere 0 is the phase shift which may be introduced by the filter i2 and the amplifier i4. From this is obtained Mlqs that of the fundamental. Since the sub-harmonic is produced by a process of modulation its amplitude cannot exceed that of the fundamental. Thus, there can be no runaway condition as in an ordinary oscillation which is limited only by the overload capacityof the oscillator. The amplitude of the sub-harmonic will be such that In the circuit of Fig. 2, the modulator is of the double balanced type disclosed in Cowan Patent No. 2,025,158, comprising a four-element copperoxide rectifier bridge 29 with the individual elements poled in the same direction, an input transformer 30 and an output transformer 3|. The secondary winding of the input transformer 30 and the primary winding of the output transformer 3| are connected respectively across the two diagonals of the bridge; and the output of the regenerative circuit 32 is connected across the mid-points of these two windings, so as to be connected to the modulator 29 in conjugate relationship with the incoming circuit through transformer 30 and the input of the amplifier connected to the output of the modulator.

The amplifier portion of the regenerative circuit 32 comprises a single amplifying vacuum tube 33 of the pentode type, having its control grid-cathode circuit connected across the secondary winding of the modulator output transformer 3!.

The anode-cathode circuit of tube 33 includes in series the primary winding of an output transformer 34 and the primary winding of the feedback transformer 35, the former primary winding being shunted by an anti-resonant circuit 35' and the latter primary winding being shunted by an anti-resonant circuit 36.

The secondary winding of the feedback transformer 35 in the regenerative circuit 32 is connected across the mid-points of the secondary winding of the modulator input transformer 30 and of the primary winding of the modulator output transformer 3!. An output circuit for taking off the converted frequency series of the feedback transformer 35 and output transformer 34, as indicated.

In the usual regenerative modulator of the type illustrated in Pig. 2, the sub-multiple frequency (where I is the input frequency) is filtered out in the amplifier and fed back to the conjugate input or the modulator. Under these circumstances there will be produced in the output of the modu later the combination frequencies Usually is eliminated by selective-circuits; however, in

thepresentcircuit,itisdesiredthatacertain amount of be applied to the modulator to combine with I to produce The products given by this secondary process are I an 51 The negative sign which appears in Equation (19) indicates that if the two frequencies,

components will be produced which are 180 degrees out of phase with each other. As long as the component indicated by Equation (19) is small compared to that of Equation (1'7), no appreclable interaction will be obtained. However, it is desirable to make the product which appears in Equation (19) as large as possible since the desired product is obtained as the other side-band.

In the circuit of Fig. 2, where it is desired to convert a frequency 1 of 4,000 cycles to 10,000

cycles the anti-resonant circuit 35' shunting the primary winding of output transformer 34 is tuned to 10,000 cycles and the anti-resonant circuit 36 shunting the regenerative or feedback circuit 32 will be tuned to approximately 2,600 cycles so that the two frequencies to be fed back, 2,000 cycles will appear on opposite sides of the resonant curve, which will give a phase displacement approaching 180 degrees.

The anti-resonant circuit 81 in series with the resistance ll across the control grid-cathode circuit of the amplifier tube N, is provided to prevent the amplifier from being over-loaded by the 2.000 cycle and 8,000 cycle waves. To accomplish this, the anti-resonant circuit 31 is preferably tuned to 10,000 cycles. The discrimination of the grid circuit against 2,000 cycles and 6,000 cycles may be varied by cha ging the value of the resistance 3B. This resistance is also convenient for adjusting the gain of the regenerative section of the amplifier at 2,000 cycles.

Some of the advantages which have been obtained with an experimental circuit substantially as shown in Fig. 2 over other known types of frequency converters are as follows: the circuit employing a single vacuum tube gave a conversion gain of approximately 20 decibels; practically the full output power of the vacuum tube is available at 10,000 cycles; the converter will not give other than the correct frequency ratio; it will remain inactive when the input is removed; and by proper design, the output may be made to vary linearly with input over a considerable range.

The circuit shown in Fig. 2 may be simplified by using a single tuned transformer to take the place of each transformer and anti-resonant circuit shown, and by using a vacuum tube of greater power capacity greater output powers may be obtained.

Fig. 3 shows a frequency converter circuit in accordance with the invention which may be used for producing accurate fractional ratios of a given frequency, for example, a

ratio (such as 4,000 to 2,400 cycles) or a ratio (such as 300 cycles to 60 cycles).

The circuit of Fig. 3 comprises a double balanced type of copper-oxide modulator 29, 30, 3|, identical with that in the system of Fig. 2 as indicated by the use of the same identifying characters, an amplifier comprising the two. amplifying vacuum tubes 43, 44, of the pentode type, an output transformer 45 for the tube 43, and a regenerative circuit comprising the duplex diodetriode unit 46 and the tuned feedback transformer 41 for feeding back the output of the amplifying tube 44 to the conjugate balanced input of the modulator.

The amplifying vacuum tube 43 comprises the heater type cathode or filament 48, the control grid 49, the suppressor grid 5| connected directly to the cathode 48, the anode or plate 50 and the screen grid 52 connected to the anode 5| through the primary winding of output transformer 45. The amplifying vacuum tube 44, which is of the variable mu type, comprises the heater type cathode 53, the control grid 54, the suppressor grid 55 connected directly to the cathode 53, the plate or anode 56 and the screen and 6,000 cycles grid 51 connected to the anode 58 through the resistance 58 and the primary winding of the transformer 59. 4

The control grid-cathode circuits of the amplifying vacuum tubes 43 and 44 are connected in parallel across the secondary winding of the modulator output transformer 31 the former circuit including in series the condenser 60 and resistance 6i and the latter circuit including in series the condenser 62 and the resistance 64. An anti-resonant circuit 64 is connected across the control grid-cathode circuit of an amplifying vacuum tube 43 and an anti-resonant circuit 65 is connected across the control grid-cathode circuit of tube 44.

The double diode or full-wave rectifier portion of the unit 46 is used to obtain a frequency multiplier whose output is nearly linear with input, and the triode part is used as an amplifier at the desired harmonic frequency to be fed back to the conjugate input of the modulator 29, 30, 3|.

The full-wave rectifier portion of the tube 46 comprises the cathode 66 of the heater type connected to the mid-point of the secondary winding of transformer 59 through a resistance 61 shunted by a resistance 68 in series with a condenser 69, and two rectifier anodes ID, H connected respectively to the two terminals of the secondary winding of transformer 59. The amplifier portion of the tube 32 comprises the oathode 66, the control grid 12 and the anode 13. The control grid 12 is connected to the mid-point of the secondary winding of transformer 59 through the condenser I5. The cathode-anode circuit of the amplifier portion of tube 46 includes the primary winding of the feedback transformer 41, which winding is shunted by the anti-resonant circuit 16.

Space current is supplied from plate battery l1 through choke coil 18 to the plates of the amplifier tubes 43 and 44 through the primary windings of transformers 45 and 59, respectively, and to the plate of the amplifier portion of tube 46 through the primary winding of the feedback transformer 4?. Appropriate heater potentials are applied from the battery "B9 to the cathode heaters of tubes 43, 44 and to the cathode heater of tube 46 through the resistance 80, and suitable fixed biasing potentials are applied to the grids of these tubes from that battery through the resistance network 89, and the resistance M, as indi cated.

The secondary winding of the feedback transformer 4? is connected acrossjthe mid-point of the secondary winding of the modulator input transformer 30 and the mid-point of the primary winding of the modulator output transformer 3| so that the feedback circuit is in conjugate relation with the incoming circuit through the input transformer 30 and with the inputs of amplifiers 43 and 44.

The cathode-control grid circuit of the variable mu amplifier tube 44 is connected to a point between the resistance 68 and condenser 69 in the common branch of the full-wave rectifier portion of tube 40, through the retard coil 82 so that the D. C. component of the rectifier provides a variaole grid bias to automatically control the gain of the variable mu tube. When an input to the variable mu tube 44 is first applied through the modulator output transformer 3i, this tube has very small control grid bias and thus a high gain for starting the regenerative action; as the regenerative component builds up to the steady state condition, the voltage drop produced by the D. C. component in the resistances 61 and 68 of the common branch of the full-wave rectifier portion of tube 44 coupled to the output circuit of the amplifier tube 44 by transformer 59 will increase the bias applied to the control grid circuit of the latter tube causing its gain to be automatically decreased.

In the circuit of Fig. 3, if the output frequency ofthe modulator is represented as f1, then the frequency out of the multiplier (output of tube 46) is rxh where r is the harmonic used. Denoting j as the input frequency to the modulator, the relation between the two frequencies is:

fitf =f (20) In the operation of the circuit of Fig. 3, the numerator of the fractional frequency obtained in the output transformer 45 will depend upon the tuning of the resonant circuit 64 in the input of amplifier 43, the denominator being determined by the submultiple frequency I: which is present in the regenerativecircuit. If a fractional ratio of for example, a conversion of 4,000 cycles to 2,400 cycles, is desired, the amplifier 43 will be tuned by resonant circuit 64 to of the input frequency (3200'cycles) by tuning the resonant circuit 16 in shunt with the feedback transformer 41 to that frequency.

There will be a considerable amount of (1600 cycles) present in the output of the duplex diode unit 32, as well as (3200 cycles) which is necessary for regeneration, and the former will modulate against f (4000 cycles) in the modulator 29, 30, 3| to produce (2400 cycles) at the output of the modulator. This will be filtered out by means of the highly selective resonant circuit 64 tuned to (2400 cycles), and will be amplified by amplifier 43 and impressed in amplified form on the outgoing circuit through output transformer 45.

Other fractional ratios; for example.

may be obtained by a suitable tuning of the resonant circuit 88.

If the condenser 89 is connected to the midpoint of transformer 89, instead of to transformer II, fractional ratios can be obtained.

Other sub-multiple ratios may be obtained by suitable tuning of the two amplifiers in the regenerative circuit. That is. if a sub-multiple ratio is' desired, the first amplifier in the regenerative circuit will be tuned to andthe second amplifier therein to and the amplifier associated with the output circuit to 3 In similarv manner fractions of can be obtained. By removing the connection to either of thediode plates I or H, odd harmonies can be obtained in the multiplier circuit thus making itposslble to secure sub-multiple ratios of If the duplex diode triode unit 48 is omitted, the circuit will give the input frequency, provided the amplifier in the feedback circuit is tuned approximately to of the input frequency.

Any other type of rectifier which is nearly linear over a considerable range may be used in place of theduplex diode triode.

The general conditions for stable operation of the type of circuit shown in Fig. 3 can be stated as follows:

(l) The total gain in the feedback circuit must be less than unity or of a non-oscillatory nature when considered as on a single frequency basis; and (2) the gain of the feedback circuit when I considered on a basis of the magnitude of f1 at Another important advantage of the circuit of Fig. 3 is that the upper limit in frequency is determined only by the ability of the circuit to modulate and amplify the frequencies in question. This limit may extend into the ultra-high frequency range.

It may be demonstrated that regenerative mod-1 ulators which employ frequency multipliers in their feedback circuits, such as illustrated in Fig. 3, usually require a finite value of thesub-multiple frequency present in the feedback circuit before they will build up to a steady-state condition. The magnitude of the sub-multiple frequency and the length of time which it must be present for purposes of starting will depend upon the amount of amplification of the amplifier and the characteristics of the frequency multiplier. Without an auxiliary starting circuit, it is necessary to make the gain of the modulator so high that it might be self-starting due to amplifier noise and circuit unbalances, and to make use of a rectifier unit which will maintain its sensitivity down to very low levels.

These requirements are dispensed with in the circuits of Figs. 4 and by providing auxiliary means for supplying the small amount of sub multiple frequency necessary for starting pur poses, so that equivalent results are obtained with much simpler circuit arrangements. The general method employed in these circuits is to supply a sharp pulse of low amplitude to one of the resonant circuits, which pulse creates a damped oscilwinding connected across one diagonal of the bridge, and an output transformer 81 having its primary winding connected across the other diagonal of the rectifier bridge. The control gridcathode circuit of a. pentode type'amplifying vacuum tube 88, similar to those illustrated in the systems of Figs. 2 and 3, is connected across the secondary winding of transformer 81. The anode-cathode circuit of tube 88 includes the primary winding 89 of the transformer 90, this winding being shunted by the resonant circuit 91 tuned to the desired sub-multiple (800 cycles) of the input frequency (4000 cycles). A second winding 82 of transformer 90 coupled to the winding 89 is provided for taking ofi' the former frequency. A third winding 93 of the transformer 90, coupled to the winding 89, is connected across one diagonal of the balanced four-element copper-oxide rectifier bridge 94 acting as a frequency multiplier, the other diagonal of the bridge being coupled by the feedback transformer 95 tuned by the resonant circuit 98 connected across its primary winding to the necessary multiple (3200 cycles) of the applied frequency (800 cycles),

former 81, as shown.

Plate current is supplied to the plate of the amthe breakdown of the neon lamp I01.

plifying vacuum tube 88 from the plate battery 91 through the retardation coil 98 and the winding 89 of transformer 90 in series. The battery 97 through the retardation coil 98 supplies a suitable bias to the screen grid of tube 88. Heating current is supplied to the heater for the cathode of tube 88 from battery 99 through the series resistance I00, and a fixed bias is supplied to the control grid of the tube 88 from that battery through the resistance-condenser network- IM and the secondary winding of the modulator output transformer 81, as shown.

The arrangement now to be described is used for producing a sharp pulse of low amplitude and applying it to the regenerative circuit to start regeneration if, for any reason, it is stopped.

The condenser I02 is arranged to becharged to the potential of the plate battery 91 through the potentiometer I03 shunting that battery and choke coil 98, the resistance I04 connected there"- to, the resistances I05 and I06 associated with the frequency multiplier unit 94, and the primarywinding of feedback transformer 95. If the regenerative modulator is not operating, the condenser I02 will charge up to the breakdown potential of the neon lamp I0 'I connected across it in series with resistance I04. The breakdown of the neon lamp I0'I discharges the condenser I02 through the resistance I04, a small partof this discharge being applied to the modulator regenerative circuit through the resistance I08 connected between a point intermediate condenser I02 and resistance I04, and one terminal of the primary winding of transformer 8'I in the input of that circuit. When the charge on the condenser I02 falls below the ionizing potential of the neon tube I0'I, it will be again charged up from battery 91 over the path previously traced.

This cycle of operations will be repeated at regular intervals to apply a series of pulses to the regenerative circuit until the regenerative modulator starts. As the regenerative modulator builds up, the amplified modulater output voltage applied from the output of amplifier 88 through transformer 90 to the rectifier bridge 94 produces a D. C. voltage drop in the resistance I06 in series with the primary winding of the feedback transformer in the output of the bridge, which voltage opposes the battery voltage applied from the plate battery 91 and, by suitable choice of the circuit constants, is made suficient to prevent Thus, when the regenerative modulator circuit is working normally, the neon lamp I0'I remains inactive, but as soon as the regenerative modulator stops operating the neon lamp will attempt to start it as described.

When the circuit is operating normally, the multiple frequency (3200 cycles) appearing in the output of the frequency multiplier unit 94 is fed by the tuned feedback transformer 95 to the conjugate terminals of the balanced modulator and modulates therein with the base frequency (4000 cycles) impressed thereon through the primary winding of the modulator input transformer 86, to produce combination frequencies in the output of the modulator which are amplified by the amplifier 88. The desired fraction of the input frequency (800 cycles) is selected by the resonant circuit 9| in the output of amplifier 88 and is impressed by transformer 90 on the utilization circuit connected to the winding 92 of that transformer, and also on the multiplier 94 through winding 93.

The circuit of Fig. 5 shows a similar starting circuit applied to a frequency converter of the regenerative modulator type in which a duplex diode-triode unit I I0, such as was described in connection with the system of Fig. 3, is used for the amplifier and multiplier in the regenerative circuit in place of the amplifier 88 and copperoxide rectifier bridge 94 employed as the frequency multiplier in the system of Fig. 4.

The starting circuit in the system of Fig. 5 also differs from that shown in Fig. 4 in the use-of a three-element, hot-cathode, gas-filled discharge tube III in place of the cold cathode neon tube I0I used in the system of the previous figure, because in the case of a duplex diode only a negative voltage can be obtained with respect to the cathode which is usually at or near ground potential. By using a three-electrode, gas-filled vacuum tube instead of the cold cathode dis charge tube, the same action may be obtained except that the voltage which is developed by the multiplier is used to control the grid of the threeelectrode, gas-filled tube.

In the system of Fig. 5, the control grid H2 and the cathode II 3 of the amplifier portion of the duplex tube I I0 are connected across the secondary winding of the modulator output transformer 81, the control grid-cathode circuit including the usual grid bias combination resistance I I4 shunted by condenser I I5, and the winding I I6 of a transformer II I is connected between the amplifier anode II 8 and the cathode II 3 of tube I I0, in series with the plate battery H9 and the choke coil I20.

The full-wave rectifier portion of the tube IIO operating as a frequency multiplier, comprises the cathode II3 connected to the mid-point of the winding I2I of the transformer 7, through the primary winding of the feed-back transformer i22, which is shunted by the tuning condenser I23, and the resistance I24 in series, and the two rectifier anodes I25, I26 connected respectively to the two terminals of the winding I 2| of the transformer Ill. The secondary winding of the feedback transformer I22 is connected across the mid-point of the secondary winding of the modulator input transformer 86 and the mid-point of the primary winding of the modulator output transformer 81.

The control grid of the gas-filled discharge tube III is connected to the mid-point of winding I2I of transformer I" through series resistances I21 and I28 so as to receive a variable direct current bias due to the voltage drop in the resistance I24 in the rectifier output of the tube IIO, and a condenser I29 is connected between the cathode of tube III and a point between the resistances I21 and I28. The resistances I30 and I 3I are connected in the anodecathode circuit of tube III in series with the plate battery H9 and choke coil I20, and the condenser I32 is connected from a point between resistances I30 and I3I to the cathode of tube I I I. The plate of tube III is connected through resistance I33 and condenser I34 in series to a point in the regenerative circuit connecting the secondary winding of feedback transformer I22 across the mid-point of the secondary winding of the modulator input transformer 86 and the midpoint of the primary winding of the modulator output transformer 81.

The heaters for the cathodes of the tubes H0 and III are supplied in series with heating current from the battery I35 through series resistance i38.

The starting circuit in the system of Fig. 5

operates as follows: The condenser I32 is charged to the potential of the plate battery H3 through the choke coil I20 and the resistance I3I. If thegegenerative modulator is not op-,

erating, the condenser I32 will charge to the breakdown potential of the gas-filled tube III, the plate-cathode circuit of which is connected "across condenser I32 in series with resistance I30.

The breakdown of the gas-filled tube discharges the condenser I32 through the resistance I30, a small part of the discharge being applied to the modulator regenerative circuit through resistance I33 and condenser I34 in series. When the charge on condenser I32 falls below the ionization potential of the tube III, that condenser will again be charged from plate battery II9 over the path previously traced.

This cycle of operations will be repeated at regular intervals to apply a series of sharp pulses to the regenerative circuit of the modulator until the regenerative modulator starts. As the regenerative modulator builds up, the amplified voltage in the output of the amplifier portion of tube III! is applied through transformer III to the output circuit of the converter, and to the double diode portion (rectifier) of tube III] and produces a D. C. voltage drop in the resistance I24 which will cause the grid voltage of the gas tube I II to become sufficiently negative to prevent the gas-filled tube III from breaking down. It will be seen, therefore, that the gas-filled tube will remain unoperated unless and until this negative voltage is built up to the point to make that tube inoperative as will occur only when the regenerative modulator circuit is operating properly, and when this voltage is not built up the gas-filled tube is operated in the manner described to attempt to start regenerative modulation.

When the regenerative modulator is working properly, a wave of a frequency which is the desired multiple of the frequency applied to the input of the double diode rectifier unit IIO (multiplier) is selected by the tuned feedback transformer I 22 and is applied to the conjugate tenninals of the modulator to modulate therein with the'base frequency wave applied to the primary Winding of the input transformer 86, to produce the desired fraction of the base frequency in the output of the modulator. This frequency is amplified by the triode portion of the tube III! and is impressed by transformer III on the utilization circuit connected to the winding I3I thereof.

Only one starting unit of the type described in connection with Fig; 4 or should be necessary for any cascaded series of regenerative modulators. That is, where several regenerative modulators are used in series, the starting unit would be controlled by the last one and the starting pulses would be applied to each regenerative modulator stage in the manner previously described for the single regenerative modulator in the previous figures. Aseach individual stage builds up, it will continue to operate regardless of the small impulses received from the starting circuit. As soon as the last unit builds up, the starting circuit will cease operating. 7

' Although the starting circuit will supply a short pulse to the output of the frequency conversion circuit, this will not be objectionable since it has been found that thispulse will be approximately 20 decibels below the normal output level, and by proper adjustment of the circuit may be made to occur not oftener than once a second. If no ineven lower because of the greater loss of the modulator. This pulse may be even made use of, if desired, to give an indication at a distant point of trouble in some preceding part of the system. For example, the operator at the distant point could by a simple test on the line determine whetherthe trouble was in the transmission line or in the regenerative modulator, according to whether the pulse was or was not heard. The lighting of the neon lamp in the case of the starting circuit of Fig. 4 would also give an indication of trouble at the point where it is located.

In frequency conversion circuits employing regenerative modulation, the amplifier merely acts as a means of supplying the necessary gain in the feedback circuit and does not become overloaded unless an extremely large input is applied to the regenerative modulator. Because of this fact, it follows that if several regenerative modulator circuits are to be used at different frequency ranges, it should be possible to utilize the same amplifier for each of the circuits, using selective networks or other means to separate the frequencies on the inputs and outputs.

A special application of this principle is in the case where several regenerative modulators are to be used in cascade. In the case of the use of second order regenerative modulators, each stage will reduce the frequency applied thereto by a factor of two.- By proper choice of the selective networks, the same amplifier may be used for several stages. Fig. 6 shows the circuit diagram of such an arrangement which was designed to convert a 2400 cycle frequency into 300 cycles and the intermediate frequencies 1200 and 600 cycles, which arrangement employs three stages of regenerative modulation and the same amplifier for each stage.

Referring to Fig. 6, it will'be seen that the regenerative modulators in the three stages are of the double balanced type, such as used in the systerms of Figs. 2 to 5, each employing a copperoxlde rectifier bridge with all the copper-oxide rectifier elements poled in the same direction, and an input and an output transformer connected respectively across the two diagonals of the bridge. The first stage of the circuit comprises a modulator including the copper-oxide rectifier bridge I38, the input transformer I 39 having its primary winding connected to the source of alternating current waves of the frequency (2400 cycles) to be converted and its secondary winding connected across one diagonal of the bridge, and the output transformer I40 having its primary winding connected across the other diagonal of the bridge I 38, a single pent-ode amplifying vacuum tube I4I including in its control grid-cathode circuit the secondary winding of transformer I 40, the combined output, feedback and interstage transformer I42 having its primary winding connected in the anode-cathode circuit of amplifier tube I4I, the anti-resonant circuit I43 in shunt with that winding, and a feedback circuit I44 connecting the secondary winding of the transformer I42 across the mid-point of the secondary winding of the modulator input transformer I39 and the mid-point of the primary winding of the modulator output transformer I 40, through the series resistances I45 and I46, as indicated.

The second stage of the circuit of Fig. 6 comprises a modulator including the copper-oxide rectifier bridge I4I, similar to the bridge I38, the transformer I42 having its secondary winding connected across one diagonal of the bridge I41 &

through the circuit I44 and the series resistances E40 and I49 and the modulator output transformer 1150 having its primary winding connected across the other diagonal of bridge I41, the am-' plifying vacuum tube I4I including in its control grid-cathode circuit the secondary winding of transformer I50 in series with the secondary winding of the transformer I45, the combined output, interstage and feedback transformer I5I having its primary winding connected in the anode-cathode circuit of the amplifier tube MI in series with the primary winding of transformer I42, the anti-resonant circuit I52 shunting that winding and the feedback circuit I53 connecting the secondary winding of transformer IfiI across the mid-point of the secondary Winding of transformer I42 and the mid-point of the primary winding of transformer I50 through the series resistances I54 and I55.

The third stage of the frequency conversion circuit of Fig. 6 comprises the modulator including the copper-oxide rectifier bridge I56, similar to the bridges I38 and I41, the transformer I5I having its secondary winding connected across one diagonal of bridge I56, through the series resistances I51 and I58, the transformer I59 having its primary winding connected across the other diagonal of the bridge I56, the amplifying vacuum tube Ml including in its control grid-cathode circuit the secondary winding of transformer I59 in series with the secondary windings of transformers I50 and l illgthe combined output and feedback transformer I60 having its primary winding comected in the anode-cathode circuit of amplifier tube Ml in series with the primary windings of the transformers I5I and I42, the anti-resonant circuit 56! shunting that winding, and the feedback circuit I62 connecting the secondary winding of transformer I60 across the mid-point of the secondary winding of transformer EM and the mid-point of the primary winding of transformer B59, through the series resistances i613 and I64.

Space current for the common amplifying vacuum tube, heating current for the heater type cathode and biasing voltages for the other electrodes thereof are supplied in a manner similar to that described for the similar tubes in the circuits of Figs. 2 to 5.

The anti-resonant circuits I4 0, i552 and EM are respectively tuned to the successively lower submultiple frequencies to be fed back to the conjugate terminals of the modulator in each stage through the feedback circuits I44, H53 and. I62, respectively, to reduce the input frequency applied to the primary winding of the modulator input transformer in the succeeding stages by a factor of two in each case.

In the case illustrated, where the wave applied to the input of the frequency conversion circuit I has a frequency of 2400 cycles, the anti-resonant circuit M0 will be tuned to select the frequency 1200 cycles from the waves in the output of amplifier Ml, and this frequency will be fed back to the conjugate terminals of the modulator in the first stage through transformer M2 and circuit MM, and will be applied also through resistances M8, M9 to the input of the modulator in the next stage,

The anti-resonant circuit I52 is tuned to select the frequency 600 cycles from the combination waves in the output of amplifier ME, which frequency will be fed back to the conjugate input of the modulator in the second stage through transformer B59 and circuit W3 and will be applied also through resistances I51 and I58 applied to the input of the modulator in the third stage.

The resonant circuit I6I in the third stage is tuned to select the frequency 300 cycles which frequency will be fed back through transformer I60 and circuit I62 to the conjugate input of the modulator in the third stage. The amplifier tube I4I will operate in each of the three stages to amplify first the applied frequencies and then the combination frequencies produced by the modulator by regeneration in each stage.

Outgoing circuits are connected to the secondary winding of transformers I42, I5I and I60 for respectively taking off the sub-multiple frequencies produced in these windings for each stage of sub-multiplication, 1200 cycles, 600 cycles and 300 cycles, as indicated.

In the circuit of Fig. 6 the balance of the modulator unit alone is utilized to prevent oscillation, whereas in each of the succeeding stages the balance of the modulator unit plus the loss of the resonant circuit in the output of the amplifier in each regenerative path is depended on to prevent oscillations in these paths. The tendency to oscillate increases as the number of stages increases. A lesser degree of balance will be required if a resonant circuit or filter tuned to the same frequency as the resonant circuit in the output of the amplifier in each stage is included in the input circuit of the amplifier for each stage. If filters in the input circuits of the amplifier are employed, the only limit to the number of stages which be used on the load carrying capacity of the amplifier and its ability to furnish the proper gain for each frequency.

In t is shown a frequency conversion circuit employing a single amplifier for a plurality of stages egenerativemodulation as in the system of and the same modulator for the several stages and other simplifications which will be pointed out below.

The first regenerative stage in the conversion circuit Z comp-rising a modulator of the double-balanced type similar to those used in the system of Figs. 2 to 6, comprising the copperoxide rectifier bridge 55 having one diagonal connected across the source of alternating current of the base frequency f to be converted (not shown) through the potentiometer I66, a regenerative circuit comprising the single amplifying pentode vacuum tube I61 and the combination tuned feedback and output transformer E60. The control grid-cathode circuit of the amplifier i61 is connected across the output diagonal of the modulator bridge I65 through the potentiometer N56. The anode-cathode circuit of the amplifying tube I61 includes the primary winding I10 of, transformer I68, this Winding being shunted by the tuning condenser I1 I. A secondary, (feedback) winding I12 of transformer i168 connects the output of the regenerative cir-,

cuit across the mid-points of the potentiometers H66 and M69 connected respectively across the input and output diagonals of rectifier bridge H65, in conjugate relation to the incoming circuit and the input of amplifier I61 in the input of the regenerative circuit. The transformer I68 has a third (output) winding I13 for taking of the converted frequency in the output of the first regenerative modulator stage. A resistance pad 814 is inserted in the regenerative circuit between the feedback winding I12 of transformer U68 and the conjugate terminals of the modulator bridge N55, to adjust the loss in the feedback circuit to a suitable value.

The second stage of the conversion circuit of Fig. 7 comprising the same modulator rectifier bridge I and associated potentiometers I66, I69, and the same amplifier l61 as used in the first stage, but a separate combined interstage, output and feedback transformer I15 and a separate feedback circuit I16.

The primary windingI11 of transformer I15, shunted by a tuningcondenser I18, is connected in the anode-cathode circuit of the amplifying vacuum tube I61 in series with the primary winding I16 of the first stage transformer I 68, shunted by condenser IN. The feedback circuit I16 of the second stage has its input connected to a second (feedback) winding I19 of transformer I15 and its output connected across the base frequency supply circuit in front of potentiometer I66. The circuit I16 includes a resistance pad I80, similar to the pad I14 in the regenerative circuit for providing a desired amount of loss in the feedback circuit I16. A third (output) winding I8I in transformer I15 is provided for taking off the converted frequency from the sec- 0nd stage.

Plate current is supplied to the plate of the amplifying tube I61 from battery I82 through winding I11 and I16 of transformer I15 and I68, in series, a positive bias potential for the screen grid of tube I61 is also obtained from plate battery I62, and a suitable negative bias on the control grid of tube I61 is obtained by usual parallel resistance-condenser biasing combination I83 in the control grid-cathode circuit of tube I 61 through the potentiometer I69. Heating current for the cathode of tube I61 may be supplied from any suitable source, not shown.

The anti-resonant circuit comprising winding I10 and parallel condenser "I in the output of tube I61 is tuned to the sub-multiple frequency and the anti-resonant circuit comprising winding I11 and parallel condenser I18 is tuned to the sub-multiple frequency In a manner similar to that described in the circuit of Fig. 6, the first stage of regenerative modulation comprising modulator I65, amplifier I61 and the regenerative circuit will operate to produce a sustained wave of the frequency which will be applied to the second stage of regenerative modulation comprising the same modulator I65, the same amplifier I61, and the regenerative circuit I16 to produce a sustained wave of the frequency which will be induced in the output winding I8I of transformer I15 connected to the load circuit, a sustained wave of the sub-multiple frequency will be induced in the other load circuit connected to the winding I13 of transformer I68.

The pad I14 in the feedback circuit will be selected to provide the proper loss to make the operation of the first stage stable, and the pad I is likewise adjusted.

If a larger amount of power in the produced waves of sub-multiple frequencies is required in the circuit of Figs. 6 and '1, two amplifying vacuum tubes may be used therein in place of the single tube illustrated, for example, in the circuit of Fig. 6, the first two stages might be arranged to make use of one of. these amplifying vacuum tubes and the last stage of the other vacuum tube. Although only three stages 01 sub-multiplication have been illustrated in Fig. 6, and two in Fig. '7, it is apparent that a greater number of stages connected in similar manner may be employed where it is desired to produce a greater step-down in frequency. The circuits of Figs. 6 and 7 as illustrated and described employ regenerative modulators of a simple second order type, but it is apparent that the same principles hold if other types of regenerative modulators are used, such as the ones illustrated in Figs. 3, 4 and 5 in which case each feedback circuit would include a suitable multiplier, for example, of the double diode-triode vacuum tube type, as of the copper-oxide bridge type, as illustrated in those figures.

Figs. 8 and 9 show frequency conversion cir cuits in accordance with the invention employing third order regenerative modulators instead of second ordermodulators as in the circuits of the previous figures.

The circuit of Fig. 8 may be employed for reducing the base frequency of ,f to the sub-multiple frequency The modulator portion of the circuit comprises the two non-linear modulating elements I84 and I85 each comprising a plate of a material of the sort hereinafter described, held between a pair of terminal electrodes, serially connected between an input transformer I86 and an output trans" former I81. The primary winding of the input transformer I86 is connected to the source of base frequency f (not shown) to be converted. The control grid-cathode circuit of a three-electrode space discharge amplifying tube I 88 is connected across the secondary winding of the transformer i81 and includes in series with that winding the usual grid-biasing arrangement comprising a resistance I89 shunted by a condenser I90. A condenser I9I connected across the secondary winding of transformer I81 forms with the inductance thereof an anti-resonant circuit. The primary winding of a combined output and feedback transformer I82 is connected in the anode-cathode circuit of tube I88 in series with the plate battery I93. The heater type cathode of tube I88 is supplied with heating current from the battery I94 as shown. The usual condenser I95 for by-passing the ac component of the output current from the plate battery is connected from the lower terminal of the primary winding of transformer I 82 to the cathode of the tube I88.

The feedback circuit I96 has its input connected across the terminals of the secondary winding of transformer I82 and its output terminals connected across the mid-point of the secondary winding of the modulator input transformer I86 and the mid-point of the primary winding of the modulator output transformer The terminals or the secondary winding of transformer I82 are also connected to a utilitill zation circuit for the converted sub-multiple frequency The plate of the non-linear modulating elements I84 and I85 in the modulator circuit of Fig. 9 preferably is of a material which comprises a mass of finely divided conductive or semi-conductive crystalline particles held together in random contact in a binding matrix of insulating substance. One example of a suitable material of this kind is a mixture of silicon carbide and carbon with clay or kaolin as a binder, as disclosed in U. S. Patent 1,822,742 issued September rear, to K. B. McEachron.

In the operation of the system of Fig. 8, the frequency f is impressed on the modulating elements its and 885 by the input transformer ills of this third order modulator, and. of the rnodulauon products appearing in the output or" the modulating elements the sub-multiple irecel a selected by the anti=resonant circuit comprisg' the secondary winding of output transformer l and the shunt condenser it i, and will be amplified by the amplifying device 38. The amplified sub -naultlple Gil s appearing in the secondary winding of the transformer will be fed back to the conjugate balanced input of the modulator by the feed-back circuit iSS and will combine in the modulator with the base l'requency i applied through input transformer I86 to produce, as indicated by Equation (6) above, the combination frequencies t ---i f. a

frequency i will be eliminated by the resonant circuit comprising the secondary winding of transformer I8?! and the shunt condenser tilt and the sustained sub-multiple frequency selected thereby will be amplified by the amplifier [188 and the amplified wave of the frequency thereof. As shown, the modulator and the succeeding amplifier in the circuit of Fig. 9 are identical with the third order modulator and the amplifier used in the system of Fig. 8, as indi cated by the use of the same characters for identifying their circuit elements. However, the transformer l9! connected to the, output or" the amplifying tube I88, in addition to a primary winding H98 in series in the cathode-anode circult of the tube I88, and a secondary winding I99 connected to the utilization circuit, has a third winding 20!] coupled to the winding I98, which is connected to the input of the double diodetriode tube 20 I, similar to the corresponding tubes used in the systems of Figs. 3 and 5 in the i'eedbackcircuit 202.

The double diode rectifier portion of the tr" 20I is utilized as a frequency multiplier, or triode portion of the tube 20! as an ad stage of amplification in the feedback circu The double diode portion of the tube till former I97 through the resonant circr't the resistance 205 in series, and. th fler anodes 208 and 20? respectively co directly to the terminals of the winding 2 The amplifier portion of the tube 5 con. or es the cathode 203, the cont grid. plate or anode 269. The control grid 208 tilt is connected to the cathode 52 -31 through i resistance 2H], and to the mid-point of windin 200 of transformer i9? ti ough the conden 2 M and resistance 24W in series.

The anode-cathode circuit the amplifier portion of the tube 20I includes in series the primary winding of the feedback transformer 2I2, the secondary winding of which is connected across the mid-point of the secondary winding of the 34 modulator input transformer I86 and the midpoint of the primary winding of the modulator output transformer i8'l, as indicated. Space current is supplied from plate battery 2I3 to the anode of amplifier tube H88 through the primary 3 winding l98 of transformer E97, and to the an ode of the amplifier portion of the tube 2E! through the primary udnding of feedback transformer 242?. Heating current is supplied from battery 2M to the lii aters for the cathodes of t tubes I88 and 2M in "es through series resistance 255.

The circuit of Fig. 9 operates as follows: wave of the base fr icy f is impressed upon the modulator circu i the input transformer 4| 286 and a desixed sub multiple frequency in the modulator output is S". ted by the anti-resonant circuit comprising the secondary winding of transformer lB'l and shunt condenser ISI. This sub-multiple frequency is amplified by the tube 6 H83 and the sub -iriultiple frequency up pearing in the winding" laid of transformer is"; is induced in the winding 200 coupled thereto, forming the i at coil for the double diode rectifier portion 0 the tube 20!, acting as a fre- 5 quency multiplier. The desired frequency multiple which is determined by the tuning of the resonant circuit 2M is impressed through condenser ?Zil upon the grid circuit of the amplifier portio o the tube 2M and is amplified thereby. 6 The alnpL led multiple frequency is impressed by feedback transformer M2 on the conjugate balanced input'of the modulator and modulates therein with the wave of base "frequency 1 impressed on the input of the modulator through 6 input transformer I86. lhe desired combination frequency in the output of the modulator is selected by the anti-resonant circuit comprising the secondary winding of transformer I81 and the shunt condenser 598. The selected compon- 7 cut is amplified by the amplifier M38 and impressed through the coils 98 and E99 of transformer G9? on the utilization circuit.

The sub multiple at which the frequency conversion circuit 02 9 operates depends upon the 7 The tuning of the two amplifiers therein, that is, the amplifier I88 and the amplifier portion of the double diode triode tube MI. 'The tuning of the first amplifier is determined by the resonant frequency of the anti-resonant circuit in the input thereof comprising secondary winding of transformer I81 and the shunt condenser I 9i, and the tuning of the second amplifier is determined by the resonant frequency of the resonant will be fed back into the conjugate input of the modulator by feedback transformer III. The frequency will combine in the modulator with the input frequency f to produce the combination frequencies s-r s The former frequency will be eliminated in the tuned input of amplifier I88 and the latter frequency,

will be selected thereby, amplified by the amplifier I88 and impressed by transformer I81 on the utilization circuit connected to the winding I99 thereof.

Other types of non-linear elements which will provide third order modulation may be substituted for the particular modulating elements I84 and I85 described for the modulators in the systems of Figs. 8 and 9, for example, magnetic devices, such as saturable core coils or transformers well known in the art.

Other modifications of the circuits illustrated and described above within the spirit and scope of the invention will occur to persons skilled in the art.

The terms balanced modulator and modulator of the balanced type as used in the claims are meant to define that type of modulator which will suppress substantially the frequency of one of the modulating waves applied to the modulat-or while transmitting other components of modulation, and the terms double balanced. modulator and modulator of the double balanced type as used in the claims are meant to define that type of modulator which will suppress substantially the frequencies of both of the modulating waves applied to the modulator while transmitting other components of modulation.

What is claimed is:

1. A frequency converting circuit for producing accurate fractions of a given base frequency comprising a modulator consisting of a plurality of solid non-linear modulating elements connected in a balanced arrangement, means for im pressing a wave of said given base frequency on the input of said modulator, a feedback circuit including an amplifier and one or more selective networks, connecting the output of said modulator to said input, in conjugate relation with the input of said amplifier, said amplifier comprising an electron space discharge device having a gain which is greater than the loss of said modulator and said networks, said selective networks being so tuned that the amplified waves fed back into the input of said modulator are of such frequency that when combined therein with the impressed wave of base frequency, sustained waves of combination frequencies including the desired frequency are produced, and means for selecting a wave of said desired frequency from the output of said amplifier.

2. A frequency converter for producing a wave of a desired frequency which is a fractional ratio of a given base frequency comprising a balanced second order modulator having one input supplied with a wave of said base frequency and a second input in conjugate relation with the first input, an amplifier having an input circuit coupled to the output of said modulator, and two output cir cuits, a regenerative circuit coupling one of said output circuits to the second input of said modulator, including means to selectively feed back to said second input waves derived from the modulator output of selected frequencies and amplitudes, which when modulated with the wave of base frequency in said modulator will produce combination waves including a sustained component of said desired frequency, and means enabling a sustained wave of said desired frequency to be selected from the other output of said amplifier.

3, The frequency converter of claim 2 in which said one output circuit is tuned so that the waves fed back to said second input of said modulator include two different frequencies of such values and amplitudes that when modulated with the supplied wave of base frequency in said modulator, a sustained side-band component of said desired frequency is produced, and said other output circuit is tuned to select said desired frequency.

4. A frequency multiplier for deriving from a wave of a base frequency f a wave of the frequency comprising a balanced modulator having one In put circuit supplied with said wave of base frequency and a second input circuit in conjugate relation with said one input circuit, a feedback circuit including an amplifier coupling the output of said modulator to said second input circuit thereof, including means to selectively feed back to said second input circuit in sufiicient amount to produce regeneration waves of the frequencies to modulate in said modulator with the wave of frequency f, and means for selecting from the resulting sustained modulation components in the output of said modulator a sustained wave of the desired frequency 5, A circuit for producing a wave of a frequency which is a desired fractional ratio of a given base 

